Effects of phase separation on stress development in polymeric coatings

A cantilever deflection technique was used to monitor stress in situ during drying of cellulose acetate coatings. Porosity was introduced in some coatings using dry-cast phase separation. Stress and weight loss profiles for dense coatings, a coating that contained small (∼1 μm) pores, and a coating that contained small (∼1 μm) pores and macrovoids (∼200 μm) are compared. In-plane tensile stress after drying ranged from 30 MPa (dense coatings) to 5 MPa (macrovoid-containing coating). The stress profiles for dense coatings feature a period of rapidly and then slowly increasing stress due to constrained shrinkage. For a coating that formed small pores, drying and stress development are delayed, stress rises and then drops a small amount due to capillary pressure relief. The stress profiles for the small pore and macrovoid-containing coatings are similar, except for a stress plateau at early stages of drying, which may be caused by macrovoid growth.